1. Field of the Invention
The present invention generally relates to wireless communications, and more particularly to use of notch filters to minimize the adverse affect of narrow band interference upon the performance of code-division multiple access systems.
2. Background Description
The worldwide market for wireless phone service is experiencing growth at an accelerated rate. It is well known that service reliability and performance are key competitive criteria in the telecommunications industry. Existing and new 800 MHz Code-Division Multiple Access (CDMA) wireless sites are in need of a solution to address problems caused by narrow band interfering signals, such as those generated by existing analog sites which continue to provide cellular coverage in urban and non-urban areas.
When deploying a CDMA telecommunications system within a frequency spectrum traditionally allocated to narrow band systems, such as the Analog Mobile Phone System (AMPS) or Global System for Mobile communications (GSM), a frequency block slightly wider than the bandwidth of the CDMA spectrum is normally cleared, in and around the geographic region of the CDMA coverage, to protect the CDMA system from narrow band interference and hence maintain the performance quality of the system. However, it is not always practicable or economical, and in some cases not possible, to clear all the required spectrum. An example where it may be impossible is in country or state boundary regions. Deleterious effects of narrow band interference on CDMA telecommunication systems include: an increase in blocked call and dropped call rates, disruption of the RF power control system, increase in average mobile station-power consumption, reduction of cell capacity, and shrinkage of cell site coverage area. In the worst case, a high level interferer can jam the entire cell or sector, blocking all CDMA communications which would normally be routed through the location.
The use of adaptive notch filters in communications is not new. U.S. Pat. No. 3,911,366 to Bagdhady describes a frequency demodulation receiver for separating a stronger and weaker signal, and removing the undesired signal. Baghdady""s invention employs a first mixer, a bandpass filter with a fixed tuned trap (notch), and a second mixer, to achieve a frequency variable notch filter. This filter bears some similarities to the frequency variable notch filter employed in this invention, however, Baghdady addresses only two FM signals and does not address scanning of a broadband signal for multiple narrowband interferors or tracking such interferors and notching them after acquisition.
U.S. Pat. No. 4,027,264 to Gutleber describes a system that scans over the spectral range of an intelligence bearing signal and locks onto and tracks the interfering signal. But the interfering signal is excised by generating a replica and subtracting that replica from the intelligence bearing signal.
U.S. Pat. No. 5,307,517 to Rich describes an improved adaptive notch filter for removing undesired cochannel FM interference, using an approach similar to that employed by Baghdady. The incoming signal is frequency translated to baseband signals and sent through a high pass filter.
U.S. Pat. No. 5,263,048 to Wade describes a method for excising narrowband interferors in a spread spectrum signal, wherein the incoming signal is digitized and transformed to the time domain and then the amplitude of the signal is discarded and replaced with a normalized value.
U.S. Pat. No. 5,703,874 to Schilling describes a spread spectrum CDMA communications system for operation within the same geographic region as occupied by a mobile-cellular system, the spectrum of which overlays the operating frequency of the mobile-cellular system. In this invention, the base station employs a comb filter for attenuating predetermined channels of the mobile-cellular system. It does not seek and attenuate narrowband transmissions at arbitrary frequencies and occurrences.
U.S. Pat. No. 5,640,385 to Long et al. describes a system for simultaneous wideband and narrowband communication, where the narrowband FM signal is embedded in the wideband signal in the base station transmitter circuitry. A notch filter is employed in the receiving circuitry and simultaneous narrowband and wideband wireless communication is supported, i.e. both the narrowband and wideband signals are recovered. Long et al. describes a system for employing both wideband and narrowband signals in an overlapping spectrum. In particular, the system embeds narrowband signals within the wideband frequency band to form a composite wideband signal, and it is important that narrowband and wideband carriers be transmitted from a common transmitter; the receiver in this system takes this composite signal, digitizes, transforms and frequency filters it so as to separate the individual narrowband and wideband signals contained in the composite. The invention provides means for optimizing the capacity of a system using such composite signals. However, Long et al. does not address the problem of conflicting systems, where a wideband system is faced with interference from narrowband signals from other systems which appear at random within the wideband spectrum.
It is therefore an object of the present invention to reduce the adverse effects of narrow band interference on CDMA communication systems, by employing an adaptive notch filter and, in wireless telecommunications applications, thereby restoring blocked call and dropped call rates, reducing or eliminating disruption of the RF power control system, avoiding increases in average mobile station power consumption, maintaining cell capacity, and maintaining cell site coverage area.
Another object of the invention is to prevent a high level interferer from jamming an entire cell or sector, blocking all CDMA communications which would normally be routed through a location.
It is a further object of the invention to provide a more convenient, less complex and less expensive method for overcoming narrow band interference.
The invention provides a device for suppressing narrow band interference in a wideband telecommunications system. Means are provided for rapidly analyzing the wide frequency band with respect to signal power levels in specified narrow frequency bands and detecting narrow band signal power levels received within the specified bands. There is shown how to derive an average composite wideband power level from signal power levels in the specified narrow bands, and then how to use these signal power levels to derive an adaptive threshold for identifying the narrow band interference. Finally, means are provided for setting one or more notch filters for suppressing the identified narrow band interference.
In accordance with the invention as applied to wideband CDMA systems, the spectrum used by the wideband CDMA signal (e.g. having a band of 1.23 MHz) is frequency scanned for narrowband analog signals (e.g. having a band of 30 KHz). An identified frequency band is then assigned to a notch filter and excised. A notable feature of the invention is that this process is fastxe2x80x94in contrast to more generic methods of filtering which take more time to be effectivexe2x80x94and appears to a telecommunications user as no more than a brief xe2x80x9cclickxe2x80x9d in reception, if at all, and excises the interferer quickly enough to prevent or greatly reduce the probability of the occurance of a blocked or dropped call due to the interference.
A further notable feature of the invention is that additional notch filters may be added to allow removal of multiple interferors. In a preferred embodiment of the invention, responsive to an environment where there are more interferors than notch filters, those interferors with the largest amplitude are assigned to the notch filters.
The present invention solves or mitigates the problem of narrow band interference in a manner which is less expensive and more convenient than other methods, and in some cases can complement other methods. The most common alternate method is to employ an Adaptive Antenna Array at the base station or mobile station. These systems are expensive and complex, and require installation of tower top antennas and other equipment
The present invention requires minimal alteration of existing base station equipment and software. Installation is simple and is not time consuming, and may be accomplished without specialized personnel because it merely requires connecting a device in the receiver RF signal path, preferably after the Low Noise Antenna (LNA). The invention dynamically detects, tracks and filters the interfering signals with sufficient speed and fidelity to eliminate or greatly reduce the deleterious effects of narrow band interfering signals on the CDMA link.
The solution requires the installation of one Adaptive Notch Filter (ANF) unit on at least one of the CDMA receivers located at each cell site. When inserted in an RF signal path the ANF detects narrow band interferors above a threshold level within the CDMA signal, and then automatically acquires and suppresses the interferors. This is achieved by electronically placing a rejection notch at the frequency of the interferors. Multiple interferors may be simultaneously suppressed depending upon the number of notch filter modules installed in the equipment.
The invention enables continuous scanning of a preset excision band to detect interferors. In the absence of interferors a bypass mode is selected allowing the RF signal to bypass the notch. Upon detection, the interferer is acquired and in accordance with the invention a switch is made to a suppression mode where the interferer is steered through the first notch section and suppressed. Multiple interferors are sorted according to level and the highest level interferors are selected and suppressed, up to the number of cascaded notch filters. Alternatively, an external control line may be used to select the bypass mode so that the signal is allowed to pass the notch section, regardless of interferer content.
The invention implements a rapid functional test which has sufficient operational alarms and metrics to allow an operator, either locally or remotely through an RS232 interface, to determine the characteristics of the interferors and averaged received composite CDMA power level Another aspect of the invention is modular construction which allows the rapid removal and replacement of functional circuit elements, i.e. power conditioner, scanner, notch filter, and operational alarms and metrics modules. The invention comprises all four modules, but may readily be expanded by the addition of notch filter modules. At a practical level, of course, if there are a large number of interferors and all of them are notched, there is a point at which there is not enough energy left in the desired signal to be decoded. While the disclosed method is specifically adapted for base station interference control, it can also be adapted to the mobile unit.
Furthermore, the invention is applicable to a variety of environments where a wideband system looks upon narrowband signals as interferors which must be excised in order to preserve the performance quality of the wideband system. The description herein uses Advanced Mobile Phone Service (AMPS) as the source of narrowband signals, but the invention applies equally to other comparable narrowband sources such as GSM (the European Global System for Mobile communications).